Hard, thin films of diamondlike carbon have been deposited on metal and glass substrates in a number of ways. By "diamondlike carbon" is meant carbon with exceptional hardness, e.g., can neither be cut with a razor blade, nor scratched by rubbing with 000 steelwool, high refractive index, e.g., .gtoreq.2.0, and high electrical resistivity. The chemical bonding appears to be dominated by sp.sup.3 tetrahedral diamondlike bonding as opposed to the sp.sup.2 trigonal bonding of graphite. Diamondlike carbon films may or may not give evidence of crystallinity by x-rays. Diamondlike carbon films also have a relatively low hydrogen content, the infrared absorption due to C-H bond-stretching at about 2900 cm.sup.-1 being absent, or relatively weak, and the hydrogen content being relatively low when measured by conventional techniques, e.g., secondary ion mass spectrometry (SIMS). Depending on deposition conditions, less hard films comprising carbon have also been deposited on metal and glass substrates. In constrast to diamondlike carbon, these films are easily scratched by a razor blade or by rubbing with steel wool, they have a lower refractive index, e.g., .ltoreq.1.5, and a relatively lower electrical resistivity. They also have a higher hydrogen content than diamondlike carbon films, the infrared absorption due to C-H bond-stretching at about 2900 cm.sup.-1 being very strong, and the hydrogen content being relatively high by SIMS.
The thermal decomposition of hydrocarbon gas, e.g., methane, has been reported to deposit carbon in the form of diamond on a diamond substrate, but no specific evidence was presented (Eversole, U.S. Pat. Nos. 3,030,187, and 3,030,188). Aisenberg, U.S. Pat. No. 3,961,103, used an ion beam of 40 eV energy to direct ionized carbon and argon at a substrate, but crystallographic studies were inconclusive. Ion impact films formed in a direct current (dc) plasma system comprising ethylene and argon are disclosed in Whitmell and Williamson, Thin Solid Films, 35 (1976), p 255, and the films comprise hard carbon on a metal cathode substrate. Films formed in a dc glow discharge plasma comprising acetylene are described by Meyerson and Smith, Journal of Non-Crystalline Solids, 35 and 36 (1980) 435-440 to comprise hydrogenated amorphous carbon. Ion impact carbon films deposited in a radio frequency (rf) generated plasma using butane at 10.sup.-1 Torr and a 180 Watt power input are described by L. Holland, U.K. Provisional Patent Application No. 33794 (1976 ) to have insulating properties. Hiratsuka, Akovali, Shen and Bell, Journal of Applied Polymer Science, Vol. 22, 917-925 (1978) polymerized methane, ethane, propane and n-butane in a plasma created by a rf glow discharge on either aluminum or sodium chloride plate and obtained C-H films. Substrate sputtering is also known to produce films of carbon on gold, aluminum and silicon, as reported by Ojha and Holland, Proc. 7th Int. Vacuum Congr. and 3rd Int. Conf. on Solid Surfaces, Vienna, 1977, p 1667. The most facile and widely applicable methods are carbon ion beam deposition and rf plasma decomposition of hydrocarbon gases. Such methods are discussed in Vora and Moravec, "Structural Investigation of Thin Films of Diamondlike Carbon" J. Appl. Phys., 52:10, pp. 6151-6157 (1981), Moravec and Lee, "Investigation of Mechanically Hard, Chemically Inert Antireflection Coatings for Photovoltaic Solar Modules", SERI Contract No. XS-0-9010-3 Progress Report, Nov. 30, 1980. 4 pp., and Holland and Ojha, "The Growth of Carbon Films With Random Atomic Structure From Ion Impact Damage In a Hydrocarbon Plasma," Thin Solid Films, 58, pp. 107-116 (1979). As reported in Vora and Moravec, films produced by these methods are observed to undergo an abrupt transition from soft (easily scratched by a razor blade) to hard (resistant to scratching by a razor blade) with increase in the ratio of rf power to deposition pressure. The value of this ratio of transistion was about 100 Watts/Torr.
In the present state of the art, thin carbon films have been successfully applied to refractory substrates, such as metal, glass and ceramic as wearresistant coatings, but the same films applied to less rigid, less thermally stable substrates such as acrylic, polyvinyl chloride (PVC), polycarbonate and other plastics fail to adhere to the surface, thus severely limiting the use of such thin films as protective coatings.
While tendency for the carbon films to separate from plastic surfaces is not wholly understood, it may be at least partially attributable to the substantial differences in coefficients of thermal expansion for hard carbon films versus plastics: Carbon (graphite) has a coefficient of thermal expansion (CTE) on the order of 1.3-1.5.times.10.sup.-6 in/in/.degree. F.; acrylics, PVC and polycarbonate have a CTE on the order of 10-50.times.10.sup.-6 in/in/.degree. F., and soda/lime glass has a CTE of about 4.8-5.1.times.10.sup.-6 in/in/.degree. F. Consequently, a means of improving the adhesion of diamondlike carbon films to relatively highly expandable/contractable and even flexible substrates, e.g., plastics, is of great interest to the protective coating art.